The invention relates to improvements in the solution polymerization of low molecular weight cyclic dichlorophosphazene oligomers to much higher molecular weight substantially linear polydichlorophosphazene polymers. More particularly, the invention relates to the use of solvents and boron catalysts in a dilute solution of substantially pure cyclic dichlorophosphazene oligomers to form substantially linear, substantially gel free, polydichlorophosphazene polymers.
The polymerization of low molecular weight cyclic dihalophosphazenes such as (NPC1.sub.2).sub.3 or (NPC1.sub.2).sub.4 to higher molecular weight linear polydihalophosphazene polymers is well known in the art.
A description of a prior process for polymerizing such cyclic dihalophosphazenes is contained in U.S. Pat. No. 4,327,064 to Fieldhouse et al., issued Apr. 27, 1982. In that patent a simplified solution polymerization process for preparing linear, high molecular weight, polydichlorophosphazene polymers is provided. A highly concentrated solution of oligomer was combined with catalyst and cycloaliphatic solvent, with the reaction allowed to proceed until maximum polymerization had occurred. While this process had certain advantages, there remained certain disadvantages. When batch processing is used, a high viscosity product solution develops at the concentrations of oligomer needed to produce high molecular weight polymer. The maximum obtainable molecular weight polymer was reduced vis-a-vis a bulk (no solvent), uncatalyzed polymerization. The concentration of cyclic oligomer in the starting mixture was far above the solubility limit at room temperature, making a slurry which could not readily be filtered, transferred or otherwise handled. The molecular weights of polymer vary widely from batch to batch depending on reaction conditions and purity of materials used. The instant invention solves each of these problems found in the prior art. It has been found that the use of the continuous reaction process of the instant invention provides a significant improvement in the attainable molecular weight, with a very high conversion rate of oligomer. In addition, the viscosity of the solution during the continuous reaction process is maintained at a level which may be easily handled.